The present invention relates generally to acoustic ink printing (AIP) and more particularly to improved print head transducers, for increasing printing uniformity.
AIP is a method for transferring ink directly to a recording medium having several advantages over other direct printing methodologies. One important advantage is, that it does not need nozzles and ejection orifices that have caused many of the reliability (e.g., clogging) and picture element (i.e., "pixel") placement accuracy problems which conventional drop-on-demand and continuous-stream ink jet printers have experienced. Since AIP avoids the clogging and manufacturing problems associated with drop-on-demand, nozzle-based ink jet printing, it represents a promising direct marking technology. While more detailed descriptions of the AIP process can be found in U.S. Pat. Nos. 4,308,547, 4,697,195, and 5,028,937, essentially, bursts of focused acoustic energy emit droplets from the free surface of a liquid onto a recording medium. By controlling the emitting process as the recording medium moves relative to droplet emission sites, a predetermined image is formed.
To be competitive with other printer types, acoustic ink printers must produce high quality images at low cost. To meet such requirements it is advantageous to fabricate print heads with a large number of individual droplet emitters using techniques similar to those used in semiconductor fabrication. While specific AIP implementations may vary, and while additional components may be used, each droplet emitter will include an ultrasonic transducer (attached to one surface of a body), a varactor for switching the droplet emitter on and off, an acoustic lens (at the opposite side of the body), and a cavity holding ink such that the ink's free surface is near the acoustic focal area of the acoustic lens. The individual droplet emitter is possible by selection of its associated row and column.
As may be appreciated, acoustic ink printing is subject to a number of manufacturing variables, including transducer piezo-electric material thickness, stress and composition variation; transducer loading effects due to wire bond attachment to the top electrode and top electrode thickness; ink channel gap control impacting acoustic wave focal point variations; aperture hole variations causing the improper pinning of the ink meniscus; RF distribution non-uniformity along the row electrodes, electromagnetic reflections on the transmission lines, variations in acoustic coupling efficiencies, and variations in the components associated with each transducer. Because of manufacturing constraints, these variables cannot be sufficiently controlled. The variables can result in non-uniform print profiles such as print head end-to-end non-uniformity printing. One type of non-uniform printing is a fixed pattern "frown" effect, wherein the intensity of ink in a middle portion of a print area is greater than at the outer edges of the print area.
A typical "frown" effect is illustrated by test print pattern A of FIG. 1. The "frown" results from non-uniform droplets, i.e., droplets that vary in size, emission velocity, emission frequency and/or other characteristics. In addition to the "frown" effect, other non-uniform printing which can occur include a "smile" effect, which exists when there is non-uniformity in printing in a direction orthogonal to the length of the print head. Non-uniform droplet ejection velocity can produce misaligned droplets. Non-uniform droplets may degrade the final image so much that the image becomes unacceptable. Therefore, a need exists to improve droplet uniformity in acoustic ink printing, for the "frown" and "smile" effects, as well as other non-uniformity patterns.